When a shaft of a fan motor used as a blower of an on-vehicle air conditioner rotates, various vibrations are produced due to a structure of the fan motor. A multiple number of excitation components occurs per rotation of the motor in response to the number of magnet poles and the number of slots of stator core of the motor. These excitation components include torque ripple and cogging torque. In the fan motor, the excitation components are transmitted via the shaft to the fan mounted to the shaft. The fan receives reaction force, produced by the reaction to the force given to the air during the rotation of the fan. This reaction force received by the fan travels via a fan fixing section to the shaft, the motor, and a housing of the air conditioner. The on-vehicle air conditioner is formed of many mechanical components including the housing, motor, and fan. Since a large number of mechanical components is assembled into the on-vehicle air conditioner, which resonates with a given frequency (hereinafter this resonance is referred to as structural resonance). When the vibration caused by the excitation components resonates with the structural resonance caused by assembling the large number of mechanical components, a noise having a peak at a given frequency is produced.
Preventive measures against transmission of the vibration have been proposed. One of the proposals is this: An attenuation member made of viscous material is disposed in order to prevent the vibration caused by a motor from traveling via a shaft to a fan. For instance, refer to Patent Literature 1, which discloses that use of a thin attenuation member prevents itself from being deformed by compressive stress when the fan is screwed to the shaft.
However, use of a conventional fan motor in an on-vehicle air conditioner encounters the following problems: Fan motors to be used in a variety of applications are formed by combining various motors with various housings. An elastic member (conventionally it is referred to as attenuation member) having a given thickness should be used for lowering a peak of noises depending on an application of the fan motor. In such a case, Patent Literature 1 teaches that the elastic member having the given thickness cannot prevent itself from being deformed by the compressive stress, or cannot prevent the vibration caused by the motor from traveling via the shaft to the fan.
Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2003-23750